A San Francisco limousine driver died after he was pinned between his vehicle and a parked car while trying to stop the limo from rolling down a hill, police said Wednesday.

Youjun Wang, 44, of Millbrae was letting passengers out of his limousine near Post and Taylor streets about 6:20 p.m. Tuesday when the car began rolling backward, said Officer Gordon Shyy, a police spokesman.

Wang stepped behind the limousine to try to stop it, but was unable to do so and was crushed between it and the parked car, Shyy said. Wang died at San Francisco General Hospital.

Investigators are trying to determine why the limousine began rolling backward, Shyy said.